Electrophotographic marking is a well-known, commonly used method of copying or printing documents. Electrophotographic marking is performed by exposing a charged photoreceptor with a light image representation of a desired document. The photoreceptor is discharged in response to that light image, creating an electrostatic latent image of the desired document on the photoreceptor's surface. Toner particles are then deposited onto that latent image, forming a toner image, which is then transferred onto a substrate, such as a sheet of paper. The transferred toner image is then fused to the substrate, usually using heat and/or pressure, thereby creating a permanent record of the original representation. The surface of the photoreceptor is then cleaned of residual developing material and recharged in preparation for the production of other images.
The foregoing broadly describes a black and white electrophotographic marking machine. Electrophotographic marking can also produce color images by repeating the above process once for each color of toner that is used to make the composite color image. For example, in one color process, called the READ IOI process (Recharge, Expose, and Develop, Image On Image), a charged photoreceptive surface is exposed to a light image which represents a first color, say black. The resulting electrostatic latent image is then developed with black toner to produce a black toner image. The recharge, expose, and develop process is repeated for a second color, say yellow, then for a third color, say magenta, and finally for a fourth color, say cyan. The various latent images and color toners are placed in a superimposed registration such that a desired composite color image results. That composite color image is then transferred and fused onto a substrate.
Color electrophotographic printing can be performed in a various ways. For example, in a single pass printer wherein multiple exposure stations image the photoreceptor during a single pass of the photoreceptor through the printer. This requires a separate charging, exposing, and developing station for each color of toner. Single pass printers are advantageous in that they are relatively fast since a composite color image is produced in one pass of the photoreceptor. Alternatively, color electrophotographic printing can be performed using only a single charging station and a single exposure station by having the photoreceptor make multiple passes through the printer.
One way of exposing a photoreceptor is to use a Raster Output Scanner (ROS). A ROS is typically comprised of a laser light source (or sources), a pre-polygon optical system, a rotating polygon having a plurality of mirrored facets, and a post-polygon optical system. In a simplified description of operation, a collimated laser beam is reflected from the facets of the polygon and passed through imaging elements that project the laser beam into a finely focused spot of light on the photoreceptor's surface. As the polygon rotates, the focused spot traces a path on the photoreceptor surface referred to as a scan line. By moving the photoreceptor as the polygon rotates the spot raster scans the surface of the photoreceptor. By modulating the laser beam with image information a predetermined latent image is produced on the photoreceptor. The plane of the sweeping beam is referred to herein as the tangential plane while the direction of motion of the photoreceptor is called the sagittal direction.
Raster output scanners are typically comprised of a number of optical elements. Unfortunately, unavoidable imprecision in the shape and/or mounting of these optical elements inevitably introduces anomalies in the quality of the scan line on the photoreceptor. One such anomaly is called bow. Bow is a deviation of a scan line in the shape of a frown or a smile. FIG. 1 illustrates two scan lines having different bows, a first scan line 6 has a "smile" shaped bow while the second scan line 7 has a "frown" shaped bow. FIG. 1 also shows an ideal scan line 5 without bow. A useful measurement for bow is the deviation between the top and the bottom of the scan line. In a monochromatic system if the bow deviation is kept below about 150 microns then the bow does not create a significant print quality problem. However, in color printing, particularly when using multiple raster output scanners, such errors seriously degrade print quality. Indeed, when multiple raster output scanners are used, if one bow forms a frown while the other forms a smile, bow errors of less than 10 microns degrade the final image. In high quality systems scan line bow should be held to about 2 microns.
Typically a bow occurs when the center ray of a light beam passing through a lens does not scan along the optical axis of the lens. The farther the center ray of the beam is from the optical axis of the lens, the greater the bow. In some raster output scanners the post polygon optical system, which typically includes multiple optical elements, introduces most of the bow. It should be noted that while it is the scan line deviations from the optical axes of the post polygon optical elements that usually produces bow, almost any optical component can introduce those deviations.
Various approaches to bow correction are in the prior art. One method is to use high quality optical systems, such systems being carefully matched when multiple raster output scanners are used. However, this approach is often prohibitively expensive, particularly when machine assembly is taken into consideration. Even then, meeting a 2 micron bow deviation requirement cannot always be met. Another approach is to add an optical element into the raster output scanner's optical system. For example, U.S. Pat. No. 5,383,047 teaches the introduction of a glass plate into the pre-polygon optical system. Rotation of that glass plate corrects for bow. However, that approach requires the introduction of another piece of glass into the optical path. Furthermore, in many raster output scanners it is the post-polygon optical system that introduces most of the bow.
In view of the detrimental effects of scan line bow, and in further view of the unavoidable imprecision in optical elements and deficiencies in prior art approaches to bow correction, a new technique of correcting scan line bow would be beneficial